The development of automobiles with low fuel consumption requires the optimization of vehicle and motor components. Here, for the energy consumption of the vehicle in the frequently occurring short-distance and city traffic the loss caused, among other things, by driving secondary power trains is particularly important. The drive performance of oil pumps, among other things, ensuring the lubrication of the motor can result in the reduction of the actual motor performance, which drastically increases the fuel consumption.
Up to 40° C. below zero, the function of the motor lubrication and a sufficiently fast motor lubrication must be ensured and during hot idling up to 160° C. the oil supply must not show any defects. The hot idling operation is characterized in a high internal leakage of the oil pump and a relatively high oil consumption of the motor. The hot idling operation is an essential operating point for sizing the oil pump.
In general, in the classical sizing of the pump the oil pump is designed for this operating point. In normal vehicle operation, this leads to an oversized oil pump, because the oil absorption line of the internal combustion engine progresses digressively over the rotation, with the characteristic pump line of the oil pump rising approximately linear in reference to the rotation. The excess supply of oil resulting therefrom is blown off via a pressure control valve in an energy consuming manner.
The above-described problem is enhanced in that the automotive industry, in particular, requests the use of oils with lower viscosity. Although this improves the function of pumps at temperatures below freezing, the volumetric effectiveness worsens at high temperatures.
Another problem is the fact that almost all pump cases are made from different materials in reference to the toothed wheelsets used. A multitude of pump cases are made from aluminum die casting, for example, for reasons of weight reduction, while the toothed wheelsets are produced from steel, in particular sintered steel. The different heat expansion coefficients of the pump case and the toothed wheelsets cause the necessarily designed end play between the toothed wheelset and the pump case to change during the increase and/or reduction of the temperature. At an increase of temperature, an approximately linear increase of the end play occurs, so that it results in an additional loss of volumetric effectiveness, which can amount to 50 to 60%. The volumetric effectiveness of a pump drops approximately linear at rising temperatures.
The above-described problem is shown in greater detail using an example of a vane cell pump with the following characteristics:
Case:aluminum - die castingWheel set:sintered steelHeight of wheel set:46 mmTemperature range:−40° C. to 150° C.Heat expansionaluminum case:0.0000238° C. − 1coefficient:Sintered steel wheelset: 0.000012° C. − 1
The end play of the pump is designed to 0.07 mm at 20° C.
Temperature difference 130° C. (20° C. to 150° C.)
Expansion of aluminum case:
46.07 mm+46.07 mm*0.0000238° C.−1*130° C.=46.213 mm
expansion of sintered steel wheelset:
46.00 mm+46.00 mm*0.000012° C.−1*130° C.=46.07 mm
This results in an end play of 0.143 mm.
Temperature difference 60° C. (−40° C. to 20° C.):
Shrinkage of aluminum case:
46.07 mm−46.07 mm*0.0000238° C.−1*60° C.=46.004 mm
Shrinkage of sintered steel wheelset
46.00 mm−46.00 mm*0.000012° C.−1*60° C.=45.967 mm
This results in an end play of 0.037 mm.
Due to the different heat expansion of the materials the end play increases at 150° C. to 0.143 mm and reduces to 0.037 mm at 40° C. Doubling the end play and reducing the viscosity of the medium leads to a loss of volumetric effectiveness by 50 to 60%. At low temperatures, due to the reduction of the end play, malfunctions can occur and result in considerable worsening of the mechanic effectiveness. An increase of end play by 0.01 mm results in approximately 1 liter/min reduction of flow at 100° C., 5.5 bar RPM (statement TV-H November 98). When designing an oil pump this volumetric loss has to be considered and the pump must be sized respectively bigger. Due to the bigger sized pump an excess supply of oil occurs at higher rotations, which has to be removed under power consumption.